Volume 217, Issue 3

Molecular crystal structures of 81 polymorphs of binary compounds are investigated with the method of coordination sequences and with the uniformity criterion for atomic sublattices. It is shown that using these tools one can estimate semi-quantitatively the relative force of inter-molecular interactions and to find so-called structure-forming lattice, i.e . the atomic sublattice, where the most stable long-wavelength phonons are likely to spread. When a crystal consists of molecules with distinguished central atoms, the polarizability of surrounding atoms is found to dictate mainly the composition of the structure-forming lattice. If the polarizability is small, crystal structure is formed by molecules as a whole, otherwise surrounding atoms form the structure basis themselves, and it is their lattice that often is one of the close packings. It is shown that if the central atom of a molecule has stereoactive lone pair the central atom can form the structure-forming lattice together with surrounding atoms.

The treatment of weak intensities in the least-squares refinement of crystal structures is a quite controver-sial subject. According to the predominant opinion, all of the collected intensities should be used in the structure refinement process. Sometimes, however, the omission of the weakest data seems to be not influent, and sometimes it can even improve the refinement results. In the present work the effects of selectively excluding reflection with low I / u ( I ) ratio from both | Fo | and Fo 2 least-squares refinements are examined for Trimethopin-Sulfametoxypyridazine 1:1 and N′-(4-methoxyphenyl)-N-phenyl-N-oxyformamidinium. An interpretation of some observed features of the results of the least-squares refinements by means of the leverage analysis is proposed.

A tiling representation and associated possible tiling flips are developed for the "unit-cluster" description of decagonal quasicrystals with reference to the example of decagonal Al 70 Co 15 Ni 15 . A perfect P1 tiling is a suitable model of the ideal quasiperiodic structure and a 180° flip of a pentagon tile can be used as an analogue of the hexagon flips in Penrose rhomb tilings. Monte Carlo simulations and calculated diffraction patterns are used to study the effect of those tiling flips on the cluster scale for some example cases. Some of these have a special importance for the quasicrystal-to-crystal transformation. Starting with the perfect P1 tiling the system is allowed to evolve using phason fluctuations whose probabilities depend on the resulting local tile configurations. The degree of order/disorder introduced and its effects on the diffraction pattern are investigated.

K 4 [Fe(CN) 5 NO 2 ]⋅KNO 2 ⋅H 2 O crystallizes in the non-centrosymmetric orthorhombic space group Bb 2 1 m (no. 36) with a = 2698.2(3), b = 1415.7(1), c = 849.6(1) pm and Z = 8 at 291 K. Due to a high degree of pseudosymmetry the deviations from centrosymmetry are small. The gross arrangement of the iron and potassium atoms is that of the AlB 2 -type, discernible from the distorted hexagons of the boron-analogous part with the composition FeK 3 while further potassium atoms are replacing Al in agreement with the formulation K 2 [FeK 3 ]. The disorder of the free nitrite ions affiliated to KNO 2 is characterized by means of additional data sets collected at temperatures of 173 K and 363 K. The results of the structure analysis are confirmed by infra-red spectroscopy. Especially the deformation modes δ (O-N-O) of the free nitrite and those of the nitro ligand show the expected frequency splitting. Mössbauer data serve as a check on chemical composition and give an independent estimate for the mean square displacement of the iron ( 57 Fe) atoms comparable to the results from X-ray diffraction. The loss of crystal water above 400 K is registered using ther-mogravimetry and differential scanning calorimetry.

A quantitative modelling of the disorder phenomena in layered NbS 2 is presented. The polytypes 2H and 3R are considered and the variations of the intensity profiles for their transformations into each other are calculated in terms of a one-dimensional disorder model.

The crystal structure of [Cu( Him ) 2 (CO 3 )]·H 2 O, determined from conventional X-ray powder diffraction methods, allowed to clarify, on the basis of a structural interpretation of the chemical transformation, the role of the title compound in the formation of the blue phase of Cu(im) 2 [Cu(Him) 2 (CO 3 )]·H 2 O is triclinic, P 1̅; a = 8.8565(7), b = 8.9579(5), c = 8.3705(6) Å, α = 105.728(4)°, β = 105.318(3)°, γ = 114.369(4)°; final R wp , R p and R F agreement factors, for 4700 data collected in the 11–105° range, are 0.053, 0.036 and 0.078, respectively.